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Variables that Never Change
Emacs Lisp has two special symbols, `nil' and `t', that always
evaluate to themselves. These symbols cannot be rebound, nor can their
value cells be changed. An attempt to change the value of `nil' or `t'
signals a `setting-constant' error.
nil == 'nil
(setq nil 500)
error--> Attempt to set constant symbol: nil
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